The present invention relates to a method for producing a molded silicone rubber product.
Compositions that comprise organohydrogenpolysiloxanes and organopolysiloxanes containing silicon atom-bonded alkenyl groups in the form of liquids, and that are formed into silicone rubber when cured in the presence of a platinum catalyst (addition reaction-curable liquid silicone rubber compositions), are used in an extremely broad range of fields to form gaskets, packing, and seals for industrial instruments and automobiles; as well as copier, fax, and printer rolls; and key pads for electronic calculators, cellular telephones, and various keyboards.
A typical method for producing molded silicone rubber products from such liquid addition reaction-curable silicone rubber compositions is to prepare two separate liquids comprising a primary agent (liquid A) containing a platinum catalyst and an organopolysiloxane containing silicon atom-bonded alkenyl groups, and a curing agent (liquid B) containing an organohydrogenpolysiloxane and an organopolysiloxane containing silicon atom-bonded alkenyl groups. The liquid A and liquid B are stored separately and mixed to form a silicone rubber-forming composition immediately before feeding the composition into a silicone rubber-molding device where the composition is heated and cured so as to produce a molded silicone rubber product.
Various inorganic fillers are frequently blended with such liquid addition reaction-curable silicone rubber compositions in order to improve the mechanical strength and heat resistance, to provide the silicone rubber with conductivity and flame retardance, and so forth. However, a common problem with liquid addition reaction-curable silicone rubber compositions containing such inorganic fillers is the poor storage stability, despite the separate storage of the two liquids described above. When these are stored for long periods of time, it is not always possible to preserve the curing properties after manufacture or the physical properties after curing. For example, when liquid addition reaction-curable silicone rubber compositions containing carbon black are stored in the form of the two separate liquids described above, the platinum catalyst activity is lost as a result of active groups on the surface of the carbon black, the adsorption capacity of the carbon black, impurities contained in the carbon black, and so forth, while the organohydrogenpolysiloxane can deteriorate as a result of condensation reaction. The liquid addition reaction-curable silicone rubber composition does not cured very rapidly, despite the division into two liquids, resulting in a cured silicone rubber that is not as hard as it should be.
Additionally, the amount of the curing agent and the amount of catalyst in the primary component cannot be independently altered when producing a molded silicone rubber product from the liquid addition reaction-curable silicone rubber composition thus divided into two types of liquids. That is, when the curing rate of the liquid addition reaction-curable silicone rubber composition or the degree of hardness of the cured silicone rubber is to be modified, however slightly, the primary component and curing agent must be entirely replaced and prepared anew, considerably compromising productivity.
The present inventors have found that the storage stability of the platinum catalyst and the curing agent can be improved, and that the period for which a silicon rubber-forming composition may be stored can thus be prolonged when a liquid addition reaction-curable silicone rubber composition is stored in the form of separate components comprising a paste primary composition comprising an inorganic filler and an organopolysiloxane containing at least 2 silicon atom-bonded alkenyl groups per molecule, a liquid catalyst composition comprising a platinum catalyst and an organopolysiloxane having at least 2 silicon atom-bonded alkenyl groups per molecule, and a curing composition comprising an organopolysiloxane with at least 2 silicon atom-bonded hydrogen atoms per molecule and an organopolysiloxane with at least 2 silicon atom-bonded alkenyl groups per molecule. The 3 liquids are mixed to produce a silicone rubber-forming composition when manufacturing a molded silicone rubber product and the composition is heated and cured to produce a molded silicone rubber product. The inventors furthermore found that such a method allows the curing rate of the liquid addition reaction-curable silicone rubber composition to be adjusted and the hardness to be adjusted, which cannot be done in conventional methods.
A method for producing a molded silicone rubber product comprising
(i) forming a mixture comprising
(A) a primary composition in the form of a paste comprising an inorganic filler and a diorganopolysiloxane containing at least 2 silicon atom-bonded alkenyl groups per molecule,
(B) a liquid catalyst composition comprising a platinum catalyst and an organopolysiloxane containing at least 2 silicon atom-bonded alkenyl groups per molecule, and
(C) a liquid curing composition comprising an organopolysiloxane having at least 2 silicon atom-bonded hydrogen atoms per molecule and an organopolysiloxane having at least 2 silicon atom-bonded alkenyl groups per molecule;
(ii) feeding the mixture comprising components (A), (B), and (C) into a molding device, and
(iii) curing the mixture comprising components (A), (B), and (C) by heating.